A 1000 watt class D amp can draw over 80 amps when driven to full power. An 8 gage wire should be fused no higher than 40 amps.
So if you're running speakers configured for 1 ohm and you want that 1000 watts, you'd damm well better have 4 gage cables. If you're only running into a single 4 ohm speaker (300 watts or so) you might get by with 8 gage.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's